The present invention relates to improvements in lift truck mounted clamping apparatus for picking up, transporting and stacking large rolls of paper such as newsprint and kraft paper. More particularly the invention relates to improvements in the geometry of paper roll-handling clamps of the pivoted arm type capable of performing the three functions of clamping a paper roll, rotating it about an axis longitudinal of the lift truck, and shifting the roll laterally from side-to-side or vertically up and down with respect to the frame by which the clamp is mounted on the lift truck mast.
In the past, it has been recognized that it is advantageous for a lift truck mounted paper roll clamp to be constructed such that the clamp arms are capable of engaging a given paper roll both in a position of equal extension, wherein the forwards ends of the clamp arms on either side of the paper roll extend a substantially equal distance forwardly of the lift truck and, alternatively, in a position of unequal extension wherein the forward end of one clamp arm extends a greater distance forwardly of the lift truck than the forward end of the other. The basic desirability of this feature is discussed, for example, in Quayle U.S. Pat. No. 2,870,929 which explains that when a paper roll is lying in a horizontal position on the floor or ground, the upper clamping arm must overreach the lower clamping arm in order to grasp the roll to pick it up. Thereafter, if the paper roll is to be stacked vertically by rotating the clamp about its axis of rotation, the overreaching which is characteristic of the position of unequal extension becomes an obstacle since both clamp arms should extend the same distance from the truck to place the vertical paper roll in close, compact proximity to other vertical paper rolls. The "unequal extension" position also makes it difficult to remove a vertical paper roll from a compact stack of vertical rolls because of the need to insert the clamp arms between the adjacent rolls. Accordingly, when handling rolls in the vertical position, the position of equal extension of the clamp arms is advantageous.
While Quayle provided a solution to the aforementioned problem by utilizing forward and rearward tilt of the mast of the lift truck in combination with tilt of the clamp frame relative to the mast, it is far more advantageous if such result can be obtained independently of such tilting, and with the use of pivoted rather than sliding clamp arms which are capable of providing a shifting function whereby the roll may be moved laterally from side-to-side or vertically up and down relative to the clamp frame to provide maneuverability of the roll with respect to the frame in tight quarters where maneuverability of the lift truck itself is limited.
Such pivoted shifting function, and the capability for moving the clamp arms to positions of equal and unequal extension respectively, have been provided by other prior art devices including those shown in Sinclair U.S. Pat. No. 3,896,957 and similar older clamps for handling cylindrical objects such as Burton U.S. Pat. No. 3,583,586 and Neale, Sr. U.S. Pat. No. 2,814,396. All of these clamps utilize a pivoted arm construction permitting both the unequal extension and equal extension positions, and also providing the shifting function. Furthermore the Sinclair and Burton patents disclose arrangements whereby the pin about which the two clamp arms pivot in unison to perform the shifting function is offset substantially from the longitudinal centerline of the lift truck and axis of rotation of the clamp. Although Sinclair states that the purpose of this offset location of the pin is to cause a paper roll to be centered with respect to the axis of rotation of the clamp when the clamp arms are in the position of unequal extension, this is actually a disadvantage since the necessary corollary of centering the roll in the unequal extension position of such a clamp is the decentering of the roll in the equal extension position. Such result is a reversal of what is actually desirable because the equal extension position is designed for handling vertically oriented rolls which, if decentered or offset transversely to the longitudinal centerline of the lift truck and rotational axis of the clamp, will always impose a sideways imbalance on both the truck and the clamp rotator mechanism in the equal extension position. The sideways imbalance, particularly when the vertical roll is being handled at a substantial height, can contribute markedly to instability and resultant sideways tipping of the lift truck, and can also place unneeded stress on the rotator mechanism. Accordingly it would be advantageous if the center of the roll were as close as possible to the longitudinal centerline of the lift truck and to the axis of rotation of the clamp when the clamp is in the equal extension position, rather than intentionally decentered by a substantial distance as taught by Sinclair. Conversely, it is advantageous to decenter the roll when the clamp is in the position of unequal extension (also the opposite of Sinclair) because the position of unequal extension is applicable to the horizontal orientation of the roll where the decentering of the roll below the axis of rotation of the clamp would in fact provide a stabilizing "pendant" effect, and where the centering of the roll would provide no particular advantage. Accordingly the decentering of the roll in the position of equal extension of the clamp arms, which in the prior art results from the offset of the pivot point about which the clamp arms move to perform the shifting function, constitutes a disadvantage to be overcome.
Of course one obvious solution would be to eliminate the offset of the pivot point about which the shifting function occurs, as in the Neale patent, making the pivot point coincident with the axis of rotation of the clamp. However the offset of the pivot point does perform a useful function in that it provides a more compact arrangement of the clamp structure, which enhances the loadlifting capacity of the lift truck, while maximizing the range of movement of the shifting function. Accordingly the problem is to maintain the offset position of the pivot point about which the shifting function occurs while at the same time minimizing the decentration of the paper roll when the clamp arms are in the position of equal extension.